Conductive gasket and material therefor

ABSTRACT

A conductive gasket and a paste for the electrical industry based on at least one elastomer ( 10 ) and on an admixture of conductive particles in the form of fibers ( 20 ), in which the fibers ( 20 ) are flexible and have been embedded in the elastomer ( 10 ) in random orientation and with formation of a large number of contact points between fibers.

BACKGROUND OF THE INVENTION

The invention relates to gaskets having electrically conductiveproperties for use in shielding the emission of electromagnetic wavesfrom electronic devices and to a conductive paste for use in theelectrical industry that can be used to form such gaskets.

Conventional electrically conductive shielding gaskets can be formedwith glass spheres and pure metallic powders and can be used forshielding at input/output (I/O) panels in electronic devices such ascomputers, to prevent leakage of electromagnetic waves and radiofrequency interference (RFI). Gaskets can be formed with a foam innerbody and conductive fabric disposed around the foam inner body. Othersare formed of conductive polymers and conductive elastomers or with foambodies coated with conductive elastomer or polymer materials. For manyfine detailed operations, it is advantages to provide the gasketmaterial in paste form.

Materials of this type are widely used and are required by a widevariety of applications. They have gained particular importance, forexample, in connection with the sealing of electromagnetically shieldedhousings in electronic devices which emit electromagnetic radiation orcan be disturbed by electromagnetic radiation penetrating from outside.To give EMI (electromagnetic interference) shielding or, respectively,RFI (radio frequency interference) shielding, and to improveelectromagnetic compatibility (EMC), the housings are produced from amaterial which is electrically conducting or has been coated with anelectrical conductor. It is known that gaskets made from an electricallyconductive flexible material can be used so that the region of thejoints at which the parts of the housing are joined together is alsogiven shielding.

An example of a material of this type is known from U.S. Pat. No.4,011,360, the contents of which are incorporated by reference. Thisknown material is based on an elastomer, specifically a silicone rubbermaterial, which has an admixture of electrically conducting particlestherein. This material polymerizes (cures) when exposed to atmosphericmoisture at room temperature.

DE 43 19 965 C2, the contents of which are incorporated herein byreference discloses the use of a material of this type for producing thehousings described above. The material is extruded as a strand of pastedirectly in the region of the joint onto one of the parts of thehousing, and is cured there to form the gasket. This process is alsoknown by the skilled worker as the formed-in-place-gasket process.

Existing gaskets, such as formed-in-place gaskets lack sufficientflexibility for certain applications. Others, can lack sufficient EMIshielding properties. Still others are difficult to use. Accordingly, itis desirable to provide an improved conductive shielding gasket and apaste for forming a gasket in place, which overcomes drawbacks of theprior art.

SUMMARY OF THE INVENTION

The invention is based on the concept of using conductive particles inthe form of fibers which are flexible. These are admixed in randomorientation with the elastomer in a way which produces a large number ofcontact points. The embedding of the fibers in the elastomer thereforetakes the form of an irregular three-dimensional matrix. The contactpoints ensure ideal and statistically random distribution conductivitywithin the material.

Depending on the nature of the fibers used, one preferred embodimentallows the selected proportion of fibers to be less than 50%, based onthe total volume of the material. It is preferably possible to reducethe proportion of fibers to less than 25% by volume. Compared withmaterials previously available, this gives a significant improvement inmechanical properties, in particular in the flexibility of thepolymerized final product, which is moreover very inexpensive.

It is preferable for the fibers used to have a diameter/length ratio ofmore than 2. Ideal results can be achieved if the diameter/length ratiois more than 10 and even more than 50. The shape of fibers of this typemakes them highly flexible, and they therefore give an ideal result withregard to the mechanical properties of the final product.

An embodiment further optimized in this respect provides for the use offibers with a diameter of less than 0.1 mm. These extremely thin fiberscan be embedded into the elastomer in an ideal manner, such that theembedded fibers have virtually no adverse effect on the flexibility ofthe elastomer.

It is also possible to design the material in a manner known as a two-or multicomponent material such as reactive materials in which a cureagent is mixed with a polymerizable or otherwise curable resin. The twocomponents are not mixed until immediately prior to processing. Thismethod can give a very inexpensive material which is easy to process.

Depending on the application, the material selected may be one whichpolymerizes (cures) at room temperature. On the other hand it is alsopossible to provide a material which polymerizes on exposure to heat, sothat the polymerization procedure can be controlled by controllingtemperature. This is of particular importance with a view to automatedmass production.

The formulation of the material is preferably such that it has lowviscosity. It is therefore particularly suitable for forming anelectrically shielding gasket for a housing, the material for the gasketbeing applied in the form of a strand directly in the region of a jointof a housing. Typical applications for a material of this type are theformation of a gasket for a mobile telephone housing or the like.However, the viscosity should not be too low, or it will run and notstay in place until cured. Typical strands of paste and gaskets formedtherefrom can be about 1-5 mm wide. In other embodiments of theinvention, the strand can be wider or thinner than this range.

It is also possible to design a material with high viscosity, forexample in order to produce sealing elements in the form of sealingstrips, sealing pads, sealing tubes or O-rings, by injection molding orextrusion.

One specific example of an application provides for the use of theconductive paste for producing a flexible gasket for anelectromagnetically shielded housing. A paste of the material is appliedby means of a path-controlled nozzle directly to a part of the housingin the region where the housing has a joint to be sealed. The nozzle ismoved over the part of the housing by means of computer-controlledmetering equipment while the plastics material is being discharged. Thevelocity of relative movement of the nozzle and the part of the housingis determined by the viscosity of the paste, the amount and velocity ofthe paste emerging from the nozzle, the cross-sectional area of thenozzle passage, the desired cross-sectional profile of the gasket to beproduced and the makeup of the material.

The strand made from the paste and dispensed in this way polymerizesunder ambient conditions at room temperature. This procedure takes arelatively long time, but can be accelerated by controlled exposure toheat or through the use of catalysts or accelerators.

The present invention provides a material of the type which can be usedas an EMI gasket. The material provided should have improved mechanicalproperties and moreover be inexpensive, in order for example to open upeven those application sectors in which economic reasons have hithertoprevented the use of large elastomer gaskets.

Accordingly, it is an object of the invention to produce a gasket andmaterials for producing the gasket which are highly conductive, highlyflexible and easy to use.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described below using the example showndiagrammatically in the figures.

FIG. 1 is a cross sectional view of a strand of paste which can be usedto form gasket material in accordance with preferred embodiments of theinvention;

FIG. 2 shows the gasket strand of FIG. 1 after deformation;

FIG. 3 shows a cross sectional view of a strand of conventional gasketmaterial; and

FIG. 4 shows the gasket strand of FIG. 3 after deformation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An elastomeric material containing conductive particles or fibers andmethod of formation is provided for use as low impedance gasketmaterials including electromagnetic interference (EMI) gaskets formedfrom the materials. The particles can be formed from Teflon®, nylon,acrylics, polyesters and other polymer fibers which can be plated,coated or otherwise covered or embedded with conductive material orotherwise rendered conductive, so as to yield highly flexibleelectrically conductive additives which can be used in the formation ofelectrically conductive gasket material.

In a preferred embodiment of the invention, the particles are in thenature of fibers of the above materials, coated or plated withelectrically conductive materials. One way of rendering the particleselectrically conductive is through an electroplating process to providethe particles with a metal surface covering and thereby provide a highlyelectrically conductive surface on the flexible particles. Suitablemetals include silver, copper, nickel or alloys of silver, copper and/ornickel, as well as other combinations and materials which provide lowimpedance surfaces for the particles. By way of nonlimiting example,preferred particles are in the range of about 10 to 100 microns inaverage diameter with a length of 20 microns to 1000 microns. In certainembodiments of the invention, the lengths and diameters of the particlescould be higher or lower than this range. In other embodiments of theinvention the length of the fibers can advantageously range from about30 microns to even several centimeters in length. Preferred particlesshould also have a low specific gravity to ensure that they do notsettle out of a paste before curing, preferably ranging from about 1.15to 1.5.

The conductive particles can be combined with compatible elastomericcompounds and formed into appropriate shielding material. Suitableelastomers include compatible silicones, foamed and unfoamedfluoropolymers, EPDM (ethylene-propylene-diene monomer) neoprene,SANTOPRENE™ and other elastomeric materials which are suitably combinedwith conductive particles/fibers as described herein for use asshielding gaskets as described herein. The conductive particle portionof the elastomeric material is advantageously from about 5% to 50% byweight of the elastomer/compound combination.

One advantageous feature of the conductive shield (gasket) is to providesuitable distribution of the conductive particle fibers, so thatsufficient conductive material is present at the surface of the gasket,to provide low impedance and high shielding. It is also important forthe particles to be in contact with each other to transmit current fromone surface to the other. Thus a large percentage of the particles mustbe in contact with other particles to present a continuous network ofconductive particles from surface to surface.

The paste (uncured material) can be used in compression molding,injection molding and extrusion processes to form preformed gaskets,sleeves, strips, o-rings, tubes and other images.

Electrically conductive particles have exhibited various shapes.Particles frequently encountered in conventional gaskets have the shapeof flakes, spheres, irregularly shaped bodies or fibers. In the simplestcase, the particles are manufactured directly from conductive material.It is moreover possible to prepare particles from nonconductivematerials and then to cover or coat these with conductive material. Ithas been determined that these particles reduce the elasticity of thecured final product. The elasticity here is given solely by theelastomer into which the particles have been embedded. Forming theparticles as flexible fiber strands can therefore yield highly flexibleconductive gaskets.

To achieve good conductivity it has generally been necessary to providemore than 50% by volume of conductive particles to ensure the requiredcontact of the particles with one another. The resultant proportion ofelastomer has been comparatively small. It has been determined that thisalso leads to a rigid final product with disadvantageous mechanicalproperties and high cost.

FIG. 3, for comparison, shows a section of a strand 1′, which iscomposed of an elastomer 10′ and of an admixture of conductive particles20′. The conductive particles 20′ have been packed tightly within theelastomer 10′ in order to ensure the required good electricalconductivity via a large number of contact points. The proportion of theparticles 20′ by volume, based on the total volume, is about 80%.

In FIG. 4, for comparison, the strand 1′ has been exposed to a force F,as arises in practice for example at a joint of a housing. Thedeformation shown diagrammatically is possible only if each of theparticles 20′ can be displaced to the side. Otherwise deformation ispossible only to a very small extent, specifically until the particles20′ have their maximum close-packed density. It has been determined thatthis can be improved by using less particles, but making the particlesin the form of longer, flexible, conductive strands.

FIGS. 1 and 2 show a strand 1 according to preferred embodiments of theinvention. A large number of long, thin, flexible fibers 20 have beenembedded in the elastomer material 10. They have random orientation, andthe fibers 20 contact one another at a large number of contact points toform a conductive network. Even without deformation, this ensureselectrical conductivity, as indicated in FIG. 1.

It can be seen directly from FIG. 2 that relatively problem-freedeformation can occur on exposure to a force F. In this case theflexibility of the strand 1 is almost exclusively a function of theflexibility of the elastomer, since the proportion by volume of thefibers 20 is less than 50%. The fibers 20 provide only slight hindranceto the deformation of the strand 1.

There is substantial freedom in the choice of fibers 20. It ispreferable to use extremely thin fibers which are relatively long andhave a cross section of less than 0.1 mm. The length is at least twicethe diameter, but the full effect of the advantage achievable is notseen until very much longer fibers are used, such as these in which thelength is 10, 50 and even more than 50 times the diameter. The diameterdata above should not be understood as a restriction implying that theonly fibers which can be used are those with approximately circularcross-section. Rather, the cross-sectional shapes which may be usedinclude others, and may per se be as desired. Diameter should beunderstood to refer to the average cross sectional side to side distanceregardless whether the particle has a round cross section.

The fibers 20 may be composed of the materials commonly used forparticles of this type, including, for example, naturally occurringmaterials. If the starting material for the fibers is nonconducting, thefibers are covered or coated with conductive material.

The strand 1 shown in FIGS. 1 and 2 has thus been permeated by a type ofthree-dimensional matrix of fibers 20, and the distribution of thefibers 20 and their contact points with one another, determinedstatistically over the cross section, is at least approximatelyconstant.

It is apparent from the above that it is possible to form a conductivepaste and an EMI shielding gasket from such paste, which has idealmechanical and electrical properties and is also inexpensive. It isrelatively easy here to achieve the desired processing properties, inparticular via suitable choice of the elastomer, which may also be inthe form of a two- or multicomponent material.

1. A conductive paste, comprising: at least one elastomeric material anda plurality of conductive particles in the form of elongated fibers,wherein the fibers are flexible and are substantially uniformlydistributed in the elastomeric material in random orientation and withformation of a large number of contact points between contacting fibers.2. The paste as claimed in claim 1, wherein the proportion of fibers,based on the total volume of the paste, is less than 50% by volume. 3.The paste as claimed in claim 1 wherein the proportion of fibers is lessthan 25% by volume of the total volume of the paste.
 4. The paste asclaimed in claim 1, wherein the fibers have a length:diameter ratio ofmore than
 2. 5. The paste as claimed in claim 1, wherein the fibers havea length:diameter ratio of more than
 10. 6. The paste as claimed inclaim 1, wherein the fibers have a length:diameter ratio of more than50.
 7. The paste as claimed in claim 5, wherein the fibers have adiameter less than about 0.1 mm.
 8. The paste as claimed in claim 1,wherein the paste is composed of two or more components which cureimmediately after they are combined.
 9. The paste as claimed in claim 1,wherein the paste comprises monomers which polymerize at roomtemperature.
 10. The paste as claimed in any of claim 1, wherein thepaste comprises monomers which polymerizes on exposure to heat.
 11. Thepaste as claimed in claim 1, wherein the paste has a sufficiently lowviscosity to be applied in the form of a strand about 1-5 mm wide,directly in the region of a joint, to form an electrically shieldinggasket for a housing.
 12. The paste as claimed in claim 1, wherein thepaste has a sufficiently high viscosity to be moldable by injectionmolding or extrusion to provide a sealing element.
 13. The paste ofclaim 1, wherein the fibers comprise metal covered polymer fibers. 14.The paste of claim 1, wherein the elastomer comprises a silicone basedelastomer.
 15. A conductive EMI shielding gasket having a top and abottom surface, comprising an elastomeric binder having elongated andflexible electrically conductive fibers substantially evenly distributedtherein and substantially each fiber is in contact with another fiber toprovide a conductive network of fibers from the top surface to thebottom surface of the gasket.
 16. The gasket of claim 15, wherein thebinder comprises a silicone based elastomer.
 17. The gasket of claim 15,constructed for use in a cellular phone.
 18. The gasket of claim 15,wherein the fibers are metal covered polymer fibers.
 19. The gasket ofclaim 15, wherein the fibers have a length to diameter ratio of overabout
 10. 20. A method of forming a gasket, comprising depositing agasket shaped strand of a curable paste of elastomeric material havingflexible conductive fibers evenly distributed therein and permitting thepaste to cure.